Radioactive materials are in widespread use in the treatment of cancer, and are also employed in photography, as tracer materials in the chemical industry and in the structural analysis of welded members. Particularly in the field of radiopharmaceuticals, a radionuclide or radioisotope must be assayed before the patient is provided with treatment. The instrument used to measure the strength or activity of the radiopharmaceutical is a dose calibrator.
The dose calibrator must be checked for accuracy prior to initial use, after each repair and periodically thereafter. The applicable tests for calibration of the dose calibrator are described in U.S. Nuclear Regulatory Guide 10.8, Appendix D, Section 2, PP. 10.8-27 to 29 (Rev. 1, October 1980). One such test is that for linearity. That is, the dose calibrator must perform accurately to within .+-.10% over the entire range of radionuclide activity contemplated during the use of the instrument. In most instances this range will be from about 500 millicuries to about 0.1 millicurie (100 microcuries), but may include activities up to 2 curies or as low as 1 microcurie.
The procedure in current use measures the initial activity of a test sample, and allows the sample to decay until the activity has decreased to the low end of the anticipated range of use, several measurements being taken through the period. Knowing the decay constant and initial activity, the actual activities of the decaying sample at various time increments may be calculated. Actual activities are then compared to measured quantities, linearity being confirmed by error deviations of less than .+-.10% for each comparison. If the initial activity is unkown, the procedure is the same except that one of the intermediate measurements is selected as a base point. Alternatively, one may calculate and compare time at incremental activities. Because the half life of technetium -99m (Tc-99m), the isotope most often used in linearity determinations, is 6.04 hours, a complete test from 500 to 0.1 millicurie takes about 74 hours. Obviously, this time consuming procedure is inconvenient at best, and may seriously delay appropriate patient treatment.